The tag cloud presents the most frequently clicked tags. The size of the font increases with popularity. I think the cloud is really there to remind people that they can tag threads with relevant keywords to help with searching, since I think it would be more useful to use the tag search than the tag cloud. For a tag search, if you start typing in entries, it should auto-fill with existing tags matching your start letters, so that can assist you in figuring out what keyword to use.